The present invention relates to an improved process for the regeneration of spent carbonaceous adsorbent which has been used for the dry desulfurization of an exhaust gas containing sulfur oxides, in particular sulfur dioxide, which comprises heating said spent adsorbent in an inert gas atmosphere.
As one attempt of dry-desulfurizing exhaust gases coming from stationary generating sources, there has been put to practical use a process of the type which comprises employing carbonaceous adsorbent and bringing it into contact with exhaust gases in a moving bed. According to this process, ammonia is sometimes introduced into the moving bed of carbonaceous adsorbent for the purpose of improving the desulfurizing efficiency of the active carbon and/or achieving the denitrification simultaneously. In any case, the dry desulfurization processes of the type mentioned above generally involves the steps of regenerating the carbonaceous adsorbent at a proper time to thereby restore its adsorbing efficiency, said carbonaceous adsorbent having been deactivated gradually by contact with exhaust gases, and then using it again for the purpose of contact with exhaust gases.
The conventional processes for the regeneration of spent carbonaceous adsorbent comprising heating the spent adsorbent in an inert gas atmosphere and regenerating may be roughly classified into two processes, that is, one wherein inert gas is itself employed as a heating medium and the other wherein sand is employed as a heating medium. The former is directed toward a process which comprises using a cross flow type moving bed regenerator and bringing a heated inert gas into direct contact with a spent adsorbent bed. The latter is directed toward a process which comprises mixing separately heated sand particles with spend adsorbent in an inert gas atmosphere. In the latter process high temperatures ranging from 500.degree. C. to 650.degree. C. are needed for regenerating the spent adsorbent. In contrast, the former process is advantageous in that comparatively low temperatures ranging from 300.degree. C. to 400.degree. C. are sufficient for regenerating the spent adsorbent because the inert gas, utilized as a heating medium, also acts as a purging gas.
Despite this, the former process is defective in that the use of a cross flow type moving bed naturally results in a temperature gradient therein and when the temperature in its low temperature zone (the upper zone of the moving bed) drops below the dew point of a corrosive desorbed gas, there is not only the possibility of the regenerator being subject to corrosion but also the disadvantage that the condensate of the desorbed gas will cause the active carbon particles to adhere to each other to form lumps. Additionally, when using the cross flow type moving bed there is needed special attention to protect the heaters and conduits used therein from the attack of the corrosive gas desorbed from the active carbon which accompanies the inert gas, i.e., the heating medium.
Further, a process for regenerating spent carbonaceous adsorbent by means of a counter current moving bed is a different embodiment of the general process of utilizing inert gas as a heating medium. This process comprises supplying a heated inert gas upwardly in relation to a bed of downwardly flowing spent adsorbent for the purpose of bringing both into contact. This process is advantageous in that it can exhibit a superior thermal efficiency. But it is defective in that the apparatus therefor is necessarily constructed to have a considerably large sectional area because the speed of the gas current must be decreased in order to prevent scattering of the fine adsorbent particles. Still further, in this process there is the possibility that the problem of dew condensation can occur due to the fact that since the outlet port of the heating gas in the moving bed is made the inlet port of spent adsorbent, the gas temperature is lowered too much. For the above reasons, the above mentioned process for regenerating spent carbonaceous adsorbent of the counter current moving bed type is not generally put to practical use.